In recent years, metallic pipelines buried in the earth (hereinafter, simply referred to as pipeline(s)) whose external surface is covered with plastic coatings having high resistivity and pipelines are being increasingly laid over long distances in parallel with high voltage AC electric power lines and/or AC powered rail transit systems, and therefore a need for measuring and assessing the AC corrosion risk of pipelines is increasing. According to analysis results of AC corrosion incidents of pipelines, it can be said that a source of AC corrosion is high voltage AC electric power lines or AC powered rail transit systems which run in parallel with the pipelines. AC corrosion occurs at a coating defect of pipelines while AC voltage is induced on the pipelines affected by these sources of AC corrosion. As such, AC current density IAC [A/m2] in the area of a coating defect is an index for obtaining AC corrosion rate.
AC current density IAC [A/m2] in the area S of a coating defect is defined as IAC=VAC/(R*S), (where, VAC: induced AC voltage on a pipeline, R: resistance to earth of a coating defect). Assuming a coating defect is circular shaped with diameter of d [m], resistance to earth of a coating defect R [Ω] is represented by R=ρ/(2d) (where, ρ: electrolyte resistivity [Ω*m]; see non-patent literature 1 shown below). Thus, the AC current density IAC [A/m2] in the area S [m2] at the coating defect can be represented by the following equation.IAC=2.26j*ω*M*I*L/[ρ*(S1/2)]Where,j: imaginary unit,ω: 2πf (f: frequency of a current flowing through an electric power line or a trolley line)M: mutual inductance between an electric power line or a trolley wire and a buried coating pipelineI: current of an electric power line or current of a trolley wireL: parallel-running distance between an electric power line or a trolley wire and a buried coating pipeline
As is apparent from the equation, the greater the induced AC voltage VAC (that is, the greater the current flow of the high voltage AC electric power lines or the greater the current flow of the trolley wires of the AC powered rail transit systems, and/or the longer the parallel running distance between the electric power lines or the trolley lines and the pipelines), the lower the resistivity ρ of the electrolyte in contact with the coating defect, and the smaller the area S of the coating defect, the larger the AC current density IAC increases, and thus the higher AC corrosion rate increases. Further, from another point of view, even if the AC voltage VAC is not that large, when the resistivity ρ of the electrolyte in contact with the coating defect is low and the area S of the coating defect is small, the AC current density IAC increases, and thus AC corrosion rate increases.
The AC current density of the coating defect in the coated pipelines buried in the earth cannot be actually measured. As such, the AC corrosion risk of pipelines is assessed by electrically connecting the coupon which is buried in the proximity of the pipelines with the pipelines and comparing two values of coupon DC current density and coupon AC current density (collectively these two values are called coupon current density) which are acquired from a measured value of current flowing through the connected wire with a cathodic protection reference which employs the coupon current density as an index. Where, the coupon simulates the coating defect of the pipelines and is a metal piece that is made of the same metal material as the pipelines having a known surface area (see below-mentioned non-patent literature 2).